The present invention generally relates to a video coding/converting apparatus. More specifically, the present invention is directed to a video coding/converting apparatus capable of converting a digital component signal into a digital composite signal.
In the case that video signals produced from a video camera, or a television camera are directly transmitted/recorded as an R (red) signal, a G (green) signal, and a B (blue) signal, corresponding to three primary colors of light, or are transmitted/recorded as three baseband signals, these signals called as "component signals". Also, when these R, G, B signals are processed in a matrix to be transmitted/recorded as baseband signals of a luminance signal Y and two color difference signals R-Y, B-Y, or when these R, G, B signals are transmitted/recorded as baseband signals of a luminance signal Y, an I signal, and a Q signal, these baseband signals are called "component signals".
In contrast thereto, when these R, G, B signals are combined (encoded) with a sync signal in accordance with a certain rule, the combined (encoded) signals are referred to as "composite signals". As this encoding system, there are proposed the NTSC system, the PAL system, and the SECAM system. In general, broadcast waves are transmitted in such a format of "composite signals".
When baseband signals such as luminance signals and color difference signals are digitally coded in the format of video signals, this digital coding is called "component coding". FIG. 1 indicates the contents of the component coding format rule recommended by CCIR (Comite' Consultatif International des Radiocommunication) in 1982.
In this format rule of FIG. 1, since the ratio of sampling frequencies with respect to the luminance signal and two types of the color difference signals are 4:2:2, this coding system is referred to as a "4-2-2 component system", which is comparable to the "2-1-1 system" in which the sampling frequencies thereof are half of the respective sampling frequencies. A digital VTR is called a "D1 machine", which is standardized by this 4-2-2 component system.
Also, as in the NTSC, PAL, SECAM systems, such signals as a luminance signal and color difference signals that have been frequency-multiplexed are coded. This coding is called "composite coding". As to this composite coding, SMPTE (Society of Motion Picture and Television Engineers) proposed in 1978 a coding rule proposal with respect to the NTSC system. FIG. 2 represents the contents of the composite coding format rule standardized by this proposal. A digital VTR is called a "D2 machine", which is standardized by this system.
As explained above, since there are electronic appliances belonging to the D1 machine series and electronic appliances belonging to the D2 machine series, digital component signals and digital composite signals are potentially required to be mutually convertible to each other. As apparent from the format rules of FIG. 1 and FIG. 2, since the sampling frequency of the digital composite signal is selected to be 14.318 MHz (=4 fsc, "fsc" being color sub-carrier frequency), whereas the sampling frequency of the luminance signal of the digital component signal is selected to be 13.5 MHz, the rate conversion of the sampling frequency is necessarily required when these signals are additionally and simply encoded/decoded.
FIG. 3 is a schematic block diagram for representing one example of the typical video coding/converting apparatus for converting the digital component signal into the digital composite signal.
In FIG. 3, reference numerals 101 and 102 indicate oversampling filters, reference numerals 103 and 104 represent rate converting circuits, and reference numeral 105 denotes an encoder circuit.
In this converting apparatus, when the digital component signal is converted into the digital composite signal, the luminance signal and the color difference signal are separately rate-converted by the rate converting circuits 103 and 104. Then, the rate-converted signals are synthesized with each other by the encoder circuit 105 and the synthesized signal is encoded, so that the digital composite signal may be produced.
However, in accordance with the above-described converting system of FIG. 3, two sets of the oversampling filters 101, 102 and two sets of the rate converting circuits 103, 104 are required with respect to these luminance and color difference signals. As a result, the entire circuit scale is increased, and the timing circuits are complex. This may cause a high-cost video coding/converting apparatus.
As previously described, in the video coding/converting apparatus for converting the digital component signal into the digital composite signal, as shown in FIG. 3, the rate converting circuits are required for the luminance signal and the color difference signal, respectively. Consequently, there are problems that this video coding/converting apparatus is made complex, large in circuit scale, and costly.